The present invention relates to improvements in voltage regulators, and is more specifically related to an improved circuit and method for regulating a voltage with respect to more than one voltage.
Most electronic systems are powered of off two voltages. Much of the circuitry in an electronic system, hereinafter first circuitry, operates at a fixed logic voltage. However, typically, some components in the logic of the electronic system, hereinafter second circuitry, need to operate when the logic voltage is not present. The start up circuitry may need to power up to start the electronic system prior to the logic voltage being available, or the shut down circuitry may need to continue to operate after the logic voltage is no longer available. For example, in a disc drive the disc parking circuitry must operate to park the head and shut down the system after the logic voltage is no longer present. Therefore, the second circuitry operates at a voltage, hereinafter input voltage, that is present in the electronic system when the logic voltage is not present and the second circuitry needs to be on. Some examples of the input voltage may be the wire voltage in an automobile or disc drive, or the back emf in a disc drive.
The input voltage may vary over time, therefore the second circuitry is operated of off the input voltage through a voltage regulator that is used to ensure that the voltage that is supplied to the second circuitry remains at a certain predetermined voltage close to the logic voltage. In current technology that predetermined voltage is typically about 5 V. However, as it becomes more common for logic circuitry to operate at lower voltages, the predetermined voltage will be adjusted lower. Ensuring that the voltage supplied to the second circuitry remains at a predetermined voltage is accomplished by comparing the voltage at the output of the voltage regulator, hereinafter output voltage, with a fixed voltage within the electronic system that is always present, such as the bandgap voltage in a disc drive.
Therefore, the voltage supplied to the second circuitry, i.e., the output voltage, tracks the fixed voltage always present in the electronic system. This ensures that the output voltage remains constant. During normal operation of the electronic system, when both the first and second circuitry are operating and both voltages are present, the first and second circuitry need to interface and work together. To work together the voltages at which the first and second circuitry operate should be equal or as close to equal as possible.
Unfortunately, neither the input voltage nor the fixed voltage is usually equal to the logic voltage, producing an output voltage that is not equal to the logic voltage. For the first and second circuitry to be able to work together correctly requires that the output voltage be adjusted to be more comparable to the logic voltage. A level shifter is usually added at each location where voltage is supplied to the second circuitry, between the voltage regulator and the input of the second circuitry where the voltage is supplied. This may require as many as thirty level shifters to be added to a typical disc drive.
Each level shifter requires several transistors. Having to add a plurality of level shifters requires a great deal of additional space, increasing both the cost and size of the circuit containing the second circuitry, going against the long standing goals of the semiconductor industry to reduce the size and cost of circuits. However, eliminating the level shifters makes the voltage supplied to the second circuitry somewhat different than the logic voltage, which is supplied to the first circuitry. This will not permit the first and second circuitry to operate well together, reducing the performance of the electronic system.